1. Field of the Invention
This invention relates to a head for an ink-jet printer.
2. Description of the Related Art
Ink-jet printers are known as one kind of terminal equipment for computers. Heads for on-demand type ink-jet printers which utilize piezoelectric elements as actuators are classified into two major types, namely into bimorph-type heads and piston-type heads.
FIG. 1 is a schematic cross-sectional view showing a bimorph-type head. The illustrated bimorph-type head has an ink chamber defined by stainless sheets 2 which are combined in layers, and a piezoelectric element 3. The piezoelectric element 3 has two electrode sheets 8a and 8b for application of voltage, and they are disposed to extend across the opposite flat faces of the piezoelectric element 3. An ink chamber 4 and a nozzle 5 communicate with each other through an ink supply chamber 6, and the ink supply chamber 6 also communicates with an ink supply channel 7. In general form, a plurality of heads each having the above-described arrangement are disposed in opposition to the surface of a recording sheet and a plurality of the nozzles 5 are therefore arranged in opposition to the same surface.
The operation of the bimorph-type head will be explained below. When a voltage is applied across the electrodes 8a and 8b of the piezoelectric element 3, the piezoelectric element 3 contracts in the transverse direction indicated by arrows 9 shown parallel to the respective electrodes 8a and 8b to deform one of the stainless sheets 2 which is attached directly to the piezoelectric element 3, thereby expelling a jet of ink droplets from the nozzle 5. Each of the heads is made to independently perform the above-described operation, thereby enabling information to be recorded.
The piston-type head will be explained below. FIG. 2 is a schematic cross-sectional view showing a particular piston-type head. The illustrated piston-type head has an ink chamber 11, a piston 12 and a piezoelectric element 13. The piezoelectric element 13 has two electrode sheets 17a and 17b for application of voltage, and they are disposed to extend across the opposite flat faces of the piezoelectric element 13. The ink chamber 11 includes a cylinder 14 in which ink is accommodated. The cylinder 14 is hermetically sealed by sealing member 18, and communicates with a nozzle 15. The cylinder 14 also communicates with an ink supply channel 16. The piston 12 and the piezoelectric element 13 are inserted into the cylinder 14 for movement in the axial direction. The piston 12 and the piezoelectric element 13 are fixed in alignment with each other. A plurality of heads each having the above-described arrangement are disposed in opposition to the surface of a recording sheet.
The operation of the piston-type head will be explained below. When a voltage which is beforehand applied across the electrodes 17a and 17b is removed, the piezoelectric element 13 expands in the lateral direction indicated by arrows 19 shown parallel to the electrodes 17a and 17b. The piston 12 is axially moved by the expansion of the piezoelectric element 13 to apply pressure to the ink in the cylinder 14, thereby expelling a jet of ink droplets from the nozzle 15. Each of the heads is made to independently perform the abovedescribed operation, thereby enabling information to be recorded.
In order to improve printing speed, it is desirable that the pitch of nozzles be made as narrow as possible so that a multiplicity of nozzles can be mounted. However, to reduce the nozzle pitch of the bimorph-type head shown in FIG. 1, if the dimension of the piezoelectric element 3 is reduced in the direction perpendicular to the surface of the sheet of FIG. 1, the amount of displacement of the piezoelectric element 3 is also reduced. If this amount of displacement is excessively reduced, it will be impossible to reliably expel a jet of ink droplets. In order to increase the amount of displacement of the piezoelectric element 3 whose dimension is reduced in the above-described manner, it may be applied a high voltage across the piezoelectric element 3. However, this method is accompanied by an increase in the cost of parts of the driving circuit.
In order to reduce the nozzle pitch of the piston-type head shown in FIG. 2, if the dimension of the piezoelectric element 13 is reduced in the direction perpendicular to the surface of the sheet of FIG. 2, the piezoelectric element 13 will be buckled due to the reduced rigidity thereof. As a result, it will be impossible to reliably expel a jet of ink droplets.
For the above-described reasons, a minimum of about 1 mm is required as the nozzle pitch of either type of head.